Testing of completed integrated circuits (chips) is an important step in their manufacture. Advantageously, initial testing is performed while the semiconductor chips are still part of the semiconductor wafer. Only those chips that operate satisfactorily are then incorporated into packages, thus saving the expense of packaging defective circuits.
As the integrated circuit art has progressed, the circuits have become higher and higher speed. It is difficult using existing probing apparatus to reliably test high-speed circuits because the inductance of the probing wires and because of inter-probe capacitance.
A number of attempts have been made to provide high-speed test probes for integrated circuits. One relatively high performance probe is described in an article entitled "Microwave Wafer Probing", published in the January, 1985, issue of Microwave Journal, p. 121. By means of a co-planar stripline, the manufacturer (Cascade Microwave) was able to maintain 50 ohms resistive impedance down very near to the test pads of an integrated circuit. In some applications this probe has acceptable electrical performance, but has serious mechanical limitations. The most severe of these mechanical problems relates to the unreliability of the contacts. Because two or more contact electrodes are formed on a single ceramic member, there is very little independent motion between the contacts. A small piece of metal from a test pad of an integrated circuit (IC) under test, which is part of a semiconductor wafer, may adhere to one of the contact surfaces. That metal prevents some electrical contacts to the test pads of the next IC to be tested because the adhering metal holds the ceramic member slightly farther away from the test pads of the next IC and thus prevents proper electrical connection thereto. By their very nature, electrical connections of this type are very precarious because a spacing of even a few atomic layers may prevent a proper contact. In addition to the problem of adhering metal, there is the problem of uneven contact wear. If a semiconductor wafer is placed into the testing apparatus in such a way that one contact has more contacting force than the other, it will wear down at a faster rate. If another semiconductor wafer is then placed into the apparatus in a slightly different way, then the more worn contact may fail to make proper electrical contact or if a small amount of metal is removed from one contact surface during cleaning, then that contact will not electrically connect to its test pad.
U.S. Pat. No. 4,116,523 (C. W. Coberly et al) is directed to a high frequency probe which includes a composite dielectric layer having a conductive metal wire formed therein and running there through. The layer is sandwiched between a pair of relatively thin conductive flexure arms. A first end of the wire is connected to a center conductor of a coaxial cable and the arms are connected by first ends thereof to the shield of the cable. Separate electrical contact points are formed on second ends of the wire and the arms. These contact points make physical and electrical contact to an electrical device being probed. This probe is represented as having a uniform electrical impedance along its length.
This probe is relatively complex and may be relatively expensive to fabricate.
It is desirable to provide probing apparatus in which there is adequate independent motion of the contacting probe points while at the same time maintaining substantially constant resistive transmission line impedance down to the test pads of the integrated circuit and which is relatively non-complex and can be fabricated with reasonable economy.